The invention relates in general to a method and apparatus for diagnosing short circuit conditions. More specifically, the invention relates to a method and apparatus for diagnosing short circuit conditions of solenoids utilized in electro-pneumatic braking systems.
Electro-pneumatic braking systems have been proposed in which electronic controls are utilized to control the operation of conventional pneumatic brakes employed on heavy vehicles such as tractor trailer rigs. FIG. 1 illustrates a proposed electro-pneumatic braking system, wherein braking commands are sent from a central control processor 14 to an electronic control unit 12 that controls the operation of a proportional valve P1 to supply air pressure from a reservoir pressure line to a brake chamber 14. A pressure sensor 16 supplies a feedback signal to the electronic control unit 12 indicative of the output pressure of the proportional valve P1. In the event of failure of the electronic control system, an isolation valve P2 is provided to switch the flow of air supplied to the brake chamber 14 from the output of the proportional valve P1 to a service pressure line of a conventional pneumatic brake control system, thereby guaranteeing that a loss of electronic control will not result in a loss of brake control. The normally closed position of the isolation valve P2 preferably connects the service pressure line to the brake chamber 14, such that any loss of signal from the electronic control unit 12 will automatically result in the isolation valve P2 reverting to the normally closed position to permit pneumatic braking control.
FIG. 2 illustrates a proposed electronic control circuit for use in the electronic control unit 12. A proportional solenoid S1 is used to control operation of the proportional valve P1. Power is supplied to the proportional solenoid S1 by device driver U2 that acts as a high side switch upon receipt of a control signal (PROP_HI_EN) received from the central control processor 10. Current flow through the proportional solenoid S1 is controlled by a transistor Q1 in response to a pulse width modulated signal (PROPORTIONAL_LO_EN) provided by the central control processor 10. An analog feedback signal (PROPORTIONAL_AN) is provided to an analog-to-digital (A/D) converter, which converts the analog feedback signal into a digital signal that can be analyzed by the central control processor 10. A status signal (HI_SIDE_ST) is also supplied to the central control processor 10 by the device driver U2.
The central control processor 10 can determine whether the proportional solenoid S1 is short circuited by analyzing the feedback signal during a diagnostic operation. If a short circuit condition exists, the control processor 10 deactivates an isolation signal (ISOLATION_HI_EN) that is normally supplied to the device driver U2 to active an isolation solenoid (not shown) that controls the operation of the isolation valve P2. Absent the isolation signal, the isolation valve P2 cuts air flow from the proportional valve P1 to the brake chamber 10, and connects the brake chamber 10 to the service line to allow the brakes to be operated solely by pneumatic control.
While the above-described operation is effective to bypass operation of the electo-pneumatic control when a short circuit condition arises, the short circuit condition may also cause damage to circuit components themselves. For example, the transistor Q1 has a limited over current capability, and can be permanently damaged if the time required to perform the diagnostic operation exceeds the over current capability limit of the transistor Q1. Such a condition can arise if the A/D converter is not ready to accept the analog feedback signal for conversion when the proportional circuit is turned on, thereby causing a delay in the performance of the diagnostic operation.
In view of the above, it is an object of the invention to provide a method and apparatus for diagnosing short circuit conditions that prevents damage to circuit components from occurring during a diagnostic operation.
The invention provides a method and apparatus for diagnosing short circuit conditions in a circuit. The apparatus includes a control processor, a control circuit coupled to the control processor, and an A/D converter coupled to the control processor and the control circuit. In operation the control processor disables operation of an A/D converter after completion of a conversion operation, enables operation of the control circuit after operation of the A/D converter has been disabled, enables operation of the A/D converter to obtain an analog signal from the control circuit after operation of the control circuit has been enabled, and disables operation of the control circuit after enabling operation of the A/D converter to obtain and convert the analog signal. The A/D converter converts the analog signal into a digital signal and the control processor analyzes the digital signal to determine if a short circuit condition exits in the control circuit. The control processor enables operation of the control circuit for a period that is less than an over current rating period of components of the control circuit, thereby insuring that the components of the control circuit will not be damaged if a short circuit condition exits.
Other advantages and features of the invention will become apparent from the following detailed description of the preferred embodiments of the invention and the accompanying drawings.